Magnetic resonance imaging system

ABSTRACT

The magnetic resonance imaging system applies a uniform static magnetic field and a gradient magnetic field to an object and further applies an excitation rotating magnetic field to cause magnetic resonance phenomena in the object, and detects the induced magnetic resonance signals and then obtains image data by these magnetic resonance signals. The system has a power controller for controlling the transmission power in a transmitter for transmitting the excitation rotating field. The system further has a transmission controller for controlling the power controller in response to the magnetic resonance signal which is received by the receiver from the object. The transmission controller controls the power controller to sequentially change the transmission power and detects the transmission power at which the maximum magnetic resonance signal is obtained in response to the reception signal by the receiver when the excitation rotating field is applied to the object, thereby controlling the power controller in accordance with the detected data.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic resonance imaging (MRI) system for imaging, by means of computed tomography (CT), the distribution of the magnetic resonance (MR) of the specific atomic nucleus existing in a specified cross section of an object (e.g., the density distribution of the nuclear spin), thereby to form an MR image.

The MRI system for a diagnosis obtains tomograms consisting of the distribution images of the MR data at the specific position of the object. The radiography principle in an example of the MRI system will now be described.

In this system, a sufficiently uniform static magnetic field H₀ is applied to object P as shown in FIG. 1 and a gradient magnetic field G_(z) is further applied to object P due to a pair of gradient coils C_(1a) and C_(1b) in addition to static magnetic field H₀. The direction of line of magnetic force of static magnetic field H₀ is parallel to the Z axis shown in the diagram. The magnetic force of gradient magnetic field G_(z) is parallel to the Z axis shown in the diagram and this gradient magnetic field has a linearly ascending gradient with respect to the direction of the Z axis (namely, the magnetic field intensity gradually differs with respect to the Z axis). The magnetic gradient of gradient magnetic field G_(z) is such that, for example, the magnetic field intensity of substantially the central portion regarding the direction of the Z axis is zero and the directions of lines of magnetic forces before and after the central portion are opposite and at the same time the magnetic field intensity gradually increases. The synthetic magnetic field of static magnetic field H₀ and gradient magnetic field G_(z) also has the magnetic gradient which is gradient with respect to the Z-axis direction, so that the magnetic field contour planes where the Z axis perpendicularly crosses are formed due to the synthetic magnetic field. Namely, in this case, the specific magnetic field contour plane (this magnetic field contour plane consists of the plane where the Z axis perpendicularly crosses) corresponds to the specific magnetic field intensity and substantial central portion regarding the Z-axis direction corresponds to the intensity of magnetic field of H₀.

The atomic nucleus resonates with static magnetic field H₀ at an angular frequency of ω₀ shown by the following expression.

    ω.sub.0 =γH.sub.0                              ( 1)

In expression (1), γ is a magnetogyric ratio which is peculiar to the atomic nucleus and is determined depending on the kind of atomic nucleus.

In addition to static magnetic field H₀ and gradient magnetic field G_(z), rotating magnetic field H₁ of the angular frequency γ₀, functions to resonate only the specific atomic nucleus, is applied to object P as a pulse through a pair of transmitter coils C_(2a) and C_(2b) provided in a probe head. This rotating field H₁ is called an "excitation pulse". Rotating field H₁ substantially selectively acts on only the portion of the X-Y plane shown in the diagram (which is selectively determined with respect to the Z-axis direction) due to the gradient magnetic field G_(z). Thus, the MR phenomenon occurs in only the specific slice portion S (although it is the plane portion, it actually has a certain thickness), from which tomograms are obtained.

Due to the occurrence of the MR phenomenon, a free induction decay (FID) signal is detected through a pair of receiver coils C_(3a) and C_(3b) provided in the probe head. This FID signal is subjected to a Fourier transformation, so that a single spectrum is derived with respect to the rotating frequency of the specific atomic nuclear spin. To obtain the tomogram, the projection data of slice portion S regarding multi directions in the X-Y plane is necessary. Therefore, after the MR phenomenon was caused by exciting slice portion S, as shown in FIG. 2, a gradient magnetic field G_(xy) having a magnetic gradient which is linear in the direction of the X' axis (coordinate axis which is rotated by an angle of θ from the X axis) is allowed to act on magnetic field Hn (by a coil or the like (not shown)). Thus, magnetic field contour lines E₁ to E_(n) in slice portion S (X-Y plane) of object P become straight lines which perpendicularly cross the X' axis. The rotating frequencies of the specific atomic nuclear spins on magnetic field contour lines E₁ to E_(n) are expressed by expression (1) mentioned above. For convenience of explanation, in this case, magnetic field contour lines E assume E₁ to E_(n) and it is possible to consider that signals D₁ to D_(n) as the FID signal are caused by the magnetic fields on those magnetic field contour lines E₁ to E_(n), respectively. Amplitudes of signals D₁ to D_(n) can be proportional to densities of the specific atomic nuclear spins on magnetic field contour lines E₁ to E_(n) which pierce slice portion S, respectively. However, the FID signal which is actually observed becomes the composite FID signal FID of which all of D₁ to D_(n) were added. This composite FID signal FID is subjected to the Fourier transformation, so that projection data (one-dimensional image) PD of slice portion S onto the X' axis is obtained. The X' axis is rotated in the X-Y plane (for example, a gradient magnetic field G_(x) for providing a magnetic gradient with regard to the X direction is caused by a pair of gradient coils, and at the same time a gradient magnetic field G_(y) for providing a magnetic gradient with respect to the Y direction is caused by another pair of gradient coils, and these gradient magnetic fields G_(x) and G_(y) are synthesized to form a gradient magnetic field G_(xy), then a synthetic ratio of gradient magnetic fields G_(x) and G_(y) is changed, thereby making it possible to realize the rotation of gradient magnetic field G_(xy)). Due to this, the projection data regarding each direction in the X-Y plane can be derived in a manner similar to the above. The MR image can be synthesized by an image reconstruction process using those projection data.

Although there is a case where the FID signal due to the magnetic resonance is observed by directly detecting the FID signal itself, there is also a case where the magnetic resonance is excited so as to generate a spin echo signal relative to the FID signal and this spin echo signal is detected, thereby enabling observation of the FID signal.

A 90° pulse and/or a 180° pulse is used as an exciting pulse in order to obtain the FID signal or spin echo signal. The 90° pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 90° from the direction parallel to the magnetic field to the direction which is normal thereto due to the resonance. Similarly, the 180° pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 180°. The 90° pulse is mainly used to obtain the sole FID signal. Both of the 90° and 180° pulses are frequently used to obtain the spin echo signal.

The conventional diagnostic MRI system encounters a problem. Even if the condition to generate the 90° or 180° pulse has been preset, the power condition of the pulse will inevitably change since, when the object if put into the transmitter coils, the quality factor (Q) and resonance frequency (f₀) of the coils alter depending on attributes of the object, for example, the shape. Thus, the angle of inclination of the magnetic moment of the spin system is deviated from the set value of 90° or 180° when the exciting pulse is applied. Consequently, picture quality features, e.g., contrast, an S/N ratio, or the like of the MR image which are obtained from the FID signal or spin echo signal, vary depending on the attributes of the object.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an MRI system of a relatively simple construction which can properly set the power condition irrespective of object attributes and can obtain a uniform magnetic resonance image.

In an MRI system according to the present invention, the excitation rotating magnetic field is transmitted while sequentially changing the transmission power to generate the excitation rotating magnetic field, and at the same time the MR signals due to the different excitation rotating magnetic fields of the transmission powers are received from the object. These received MR signals are discriminated and the transmission power at which the maximum received MR signal is obtained is detected. The power condition of the transmitting pulse is controlled in accordance with the value of this transmission power such that the angle of inclination of the magnetic moment of the spin system becomes a predetermined value independently of the attributes of the object.

According to the MRI system of the invention, the optimum power condition is set irrespective of object attributes, so that uniform magnetic resonance images can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the principle of an MRI system;

FIG. 2 is a diagram for explaining the principle to obtain the projection data due to the MR phenomenon;

FIG. 3 is a block diagram showing a schematic arrangement of an MRI system according to an embodiment of the invention;

FIG. 4 is a block diagram showing a detailed arrangement of a transmitting section in the system of FIG. 3; and

FIG. 5 is a block diagram showing a detailed arrangement of a power controller in the arrangement of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows a schematic arrangement of an MRI system according to an embodiment of the present invention. In FIG. 3, a very simplified diagram of the arrangement of the system is shown.

This MRI system comprises, as a fundamental arrangement, transmitting pulse controller 1, transmitter 2, transmitter coil 3 (corresponding to transmitter coils C_(2a) and C_(2b) shown in FIG. 1), receiver coil 4 (corresponding to receiver coils C_(3a) and C_(3b) shown in FIG. 1), and receiver 5. (It is obvious that in addition to those components, a construction to apply a static magnetic field and gradient magnetic fields to an object is also provided although it is not shown.)

Controller 1 includes, for example, a CPU (central processing unit) which performs the control operation. Transmitting pulse controller 1 changes the transmitting pulse power which is supplied to transmitter coil 3 for generating an excitation rotating magnetic field in accordance with the attributes of an object (not shown) in a manner such that an angle of inclination of the magnetic moment of the spin system becomes a predetermined value. Practically speaking, for example, in the case of receiving a spin echo signal from the object through receiver coil 4, an excitation rotating magnetic field H₁ is applied to the object while preliminarily sequentially changing the transmitting pulse power within a predetermined range, thereby obtaining the value of the transmitting pulse power at which the peak value of the received echo signal becomes maximum. The transmitting pulse power when the object is radiographed is automatically set in accordance with this power value.

Transmitter 2 will then be described in detail with reference to FIG. 4 showing its arrangement.

As shown in FIG. 4, transmitter 2 comprises: oscillator 6; modulator 7; selector 8; power controller 9; first amplifier 10; second amplifier 11; third amplifier 12; impedance matching circuit 13; and tuner 14.

Oscillator 6 generates a continuous wave. Modulator 7 amplitude-modulates, by pulse wave, the continuous wave output from oscillator 6. Selector 8 is used to select an amplitude and/or a width of the pulse wave serving as a modulating wave in modulator 7. Power controller 9 controls an output of modulator 7 in response to controller 1 and eventually variably controls the transmitting pulse power. First to third amplifiers 10 to 12 constitute a radio frequency (RF) amplifier consisting of three stages. First amplifier 10 amplifies an output of power controller 9. Second amplifier 11 amplifies an output of first amplifier 10. Third amplifier 12 as the last stage amplifier amplifies an output of second amplifier 11. An output of third amplifier 12 is connected to tuner 14 and transmitter coil 3 through impedance matching circuit 13. Tuner 14 is used to tune transmitter coil 3.

Power controller 9 will then be explained in detail with reference to FIG. 5 showing its arrangement.

As shown in FIG. 5, power controller 9 comprises: differential receivers 15 and 16; a latch circuit 17; a D/A (digital-to-analogue) converter 18; and an attenuator 19.

Differential receivers 15 and 16 receive (digital) control signals which are transmitted from transmitting pulse controller 1. Receivers 15 and 16 receive high order bits and low order bits of the control signals, respectively. Latch circuit 17 latches the control signals which are outputted from receivers 15 and 16 in response to a timing signal which is supplied by controller 1. D/A converter 18 converts the digital signal latched in latch circuit 17 to an analog signal. Attenuator 19 is interposed between modulator 7 and first amplifier 10 and its impedance varies depending on an output of D/A converter 18, thereby changing a signal which is supplied from modulator 7 to first amplifier 10. Namely, attenuator 19 controls an output current of modulator 7 and the resultant output which was current controlled is inputted to first amplifier 10.

The operation of the system constituted in this manner will then be described.

Before starting radiographing an object, the transmitting pulse power is set by controller 1 in the following manner.

When the system is installed, the condition to generate the 90° or 180° pulse if adjusted manually. This condition can be automatically controlled to provide the correct 90° or 180° pulse as will be explained below.

Upon examination, controller 1 first supplies a control signal to transmitter 2 and transmitter 2 operates in response to this control signal, so that the excitation rotating magnetic field is applied to the object put in transmitter coil 3. The spin echo signal which is generated from the object is received by receiver 5 through receiver coil 4 and input to controller 1.

Controller 1 detects and holds the peak value of the input spin echo signal and simultaneously varies attenuator 19 in power controller 9 to change the transmitting pulse power and applies the excitation rotating magnetic field to the object in a manner similar to the above. Then, controller 1 holds the peak value of the spin echo signal in association with the change of the transmitting pulse power.

In this manner, on the basis of a plurality of spin echo signals collected by changing the transmitting pulse power, controller 1 recognizes the transmitting pulse power at which the peak value of the spin echo signal becomes maximum, namely, the control state of attenuator 19 in power controller 9. In accordance with this control state, the transmitting pulse power according to the attribute of the object is set. In other words, even when an object is put in transmitter coil 3 and the value of Q varies due to the attributes of the object, the power condition to generate the 90° pulse or 180° pulse is properly set in accordance with the attribute of the object. In this way, an angle of inclination of the magnetic moment of the spin system due to the excitation rotating magnetic field is accurately adjusted to the predetermined value of 90° or 180°.

After completion of the adjustment as mentioned above, the excitation rotating magnetic field is applied to the object by the transmitting pulse power set and the MR signals are collected from the object through receiver coil 4.

The MR signals collected from the object are subjected to an image reconstituting process, so that a distribution image of the MR data is obtained. This image reconstituting process is similar to the process in the conventional system; therefore, its description is omitted.

As described above, this system is provided with transmitting pulse controller 1 for setting an angle of inclination of the magnetic moment of the spin system to a predetermined value by controlling the transmitting pulse power which is used to generate the excitation rotating magnetic field in accordance with the attribute of the object. The object is radiographed with the transmitting pulse power which was automatically set in accordance with the transmitting pulse power at which the peak value of the received echo signal from the object is maximised by transmitting pulse controller 1. Therefore, this system doesn't ignore quality factor Q which varies depending on the shape or the like of the object as in the existing system, so that the power condition can be suitably set independently of the attributes of the object and uniform magnetic resonance images can be obtained. Also, since the MR signal, e.g., spin echo which is detected by the receiving system for use in the inherent MR measurement is detected and controlled, the arrangement of the apparatus is also rational and the apparatus doesn't become more complicated than necessary.

Although the invention has been described with respect to a preferred embodiment, the invention is not limited to the foregoing embodiment, various changes and modifications which are obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.

For example, in the system in the foregoing embodiment, attenuator 19 in power controller 9 may be changed to vary the amplitude of the transmitting pulse, thereby controlling the transmitting pulse power. However, since the transmitting pulse power changes due to the amplitude and/or width of the transmitting pulse, the amplitude and/or width of the modulating pulse may be changed by selector 8 shown in FIG. 4 and the pulse amplitude and/or pulse width of the modulating wave (output of oscillator 6) is changed, and the transmitting pulse power can be also changed. Therefore, in this case, transmitting pulse controller 1 sets the transmitting pulse power through selector 8. With this method, an effect similar to that in the foregoing embodiment can be also obtained.

Further, although the system in which the spin echo signal is received has been described in the above embodiment, the invention can be also applied to a system in which the FID signal is received in a manner similar to the above.

The present invention is not limited to the system described above, wherein the projection reconstruction method is employed. It can be applied to an MR imaging system which uses so-called Fourier transformation method such as the spin warp method, wherein excitation pulses are applied to an object, a magnetic field inclined in a specified direction is applied to the object, thus phase-modulating MR signals, and the modulated signals are Fourier-transformed. 

What is claimed is:
 1. A magnetic resonance imaging system comprising:magnetic field generating means for generating a static magnetic field and a gradient magnetic field which are applied to an object; means for applying an excitation rotating magnetic field to excite a magnetic resonance in said object to which said static magnetic field and said gradient magnetic field have been applied, said rotating field applying means having power control means for controlling a power of the excitation rotating magnetic field; means for receiving a magnetic resonance signal due to the magnetic resonance which is caused in the ooject due to said static magnetic field, said gradient magnetic field, and said excitation rotating magnetic field; image reconstruction processing means for reconstructing a magnetic resonance image from the magnetic resonance signal which is received by said receiving means; and control means for controlling said power control means in response to the magnetic resonance signal which is received by said receiving means.
 2. A magnetic resonance imaging system according to claim 1, wherein said control means has means for detecting transmission power data when the maximum magnetic resonance signal is received by said receiving means; applies the excitation rotating magnetic field to the object while sequentially changing the transmission power by controlling said power control means; detects the transmission power at which the maximum magnetic resonance signal is obtained in response to a reception signal which is obtained by said receiving means at this time; and controls said power control means in accordance with said detection data.
 3. A magnetic resonance imaging system according to claim 1, wherein said control means is means which responds to a spin echo signal which is produced due to the magnetic resonance.
 4. A magnetic resonance imaging system according to claim 1, wherein said control means is means which responds to an FID signal which is produced due to the magnetic resonance.
 5. A magnetic resonance imaging system according to claim 1, wherein said power control means is means for controlling an angle of inclination of a spin system due to the magnetic resonance in said object to a predetermined value.
 6. A magnetic resonance imaging system according to claim 5, wherein said predetermined value of the angle of inclination of said spin system is 90°.
 7. A magnetic resonance imaging system according to claim 5, wherein said predetermined value of the angle of inclination of said spin system is 180°.
 8. A magnetic resonance imaging system according to claim 5, wherein said predetermined value of the angle of inclination of said spin system is 90° and 180°.
 9. A magnetic resonance imaging system according to claim 1, wherein said power control means is means for controlling an amplitude of a signal to generate the excitation rotating magnetic field.
 10. A magnetic resonance imaging system according to claim 1, wherein said power control means is means for controlling a time width of a signal to generate the excitation rotating magnetic field.
 11. A magnetic resonance imaging system according to claim 1, wherein said power control means is means for controlling an amplitude and a time width of a signal to generate the excitation rotating magnetic field. 